The present invention relates to a self-starting type permanent magnet synchronous motor and a compressor using the same.
The advantage of an induction motor lies in its robust structure, and in that the induction motor allows full voltage starting with a commercial power source and therefore can be constructed at low cost as the driving source of a constant speed drive machinery that does not require speed control.
The self-starting type permanent magnet synchronous motor also allows full voltage starting with a commercial power source, as with the induction motor, and thus can constitutes the driving section without adding an inverter. Furthermore, since the secondary copper loss during steady operation becomes minimal, the self-starting type permanent magnet synchronous motor can significantly contribute to increase the efficiency of the driver system as compared with the induction motor.
On the other hand, one of the disadvantages of the self-starting type permanent magnet synchronous motor is that a permanent magnet is arranged on the inner peripheral side of a cage type coil and therefore a magnetic flux axis of a rotor is already fixed. That is, starting torque occurring at the rotor during start-up is a combination of both an induction torque occurring at the cage type coil and an attraction between the permanent magnet flux and a stator flux generated by an application of power. In the full voltage starting with a commercial power source, the rotor position cannot be identified (voltage phase cannot be controlled) unlike in driving an inverter motor. Accordingly, depending on the phase of the voltage applied during start-up, the magnet flux and the stator flux may repel to each other or the rotor may be attracted in the opposite direction of the normal rotating direction, and a negative torque may be generated. Thus, the self-starting type permanent magnet synchronous motor has a problem that a significant difference occurs in the torque during start-up depending on the phase of an applied voltage, i.e., on the position where the stator flux occurs.
Conventionally, a specific solution for such difference in the torque during start-up has not been proposed, although JP-A-7-298578 discloses a means for suppressing even order harmonic contents, in particular the lower order harmonic contents, generated from the stator and suppressing a fluctuation in the torque.
As described above, during start-up of the self-starting type permanent magnet synchronous motor, the difference in the starting torque occurs depending on the phase of an applied voltage. The reason for this and the problem associated therewith are described hereinafter.
When the stator flux generated by application of a supply voltage occurs on a lagging side relative to the normal rotating direction with respect to the permanent magnet flux, a magnet torque attracted in the direction opposite to the normal rotating direction will occur at the rotor. The rotor moves to the negative rotating direction because it is rotatably supported by a bearing. In this case, the rotating magnetic field of the stator is rotating in the normal rotating direction, and therefore, from the viewpoint of the slippage-torque characteristic of the induction motor, the initial move starts from a region having a slippage factor of no less than one. Accordingly, an excessive induction torque with respect to a desired value will occur.
This may have significant adverse effects, such as that an excessive stress is applied to a bearing of the motor, shortening the life time of the bearing, or that a large torsional stress is applied to the equipment attached to the end of the output axis and this equipment leads to destruction.
On the other hand, when the stator flux generated by application of a supply voltage occurs on an advance side relative to the normal rotating direction with respect to the permanent magnet flux, a magnet torque in the normal rotating direction occurs at the rotor. Therefore, the effect on the induction torque occurring at the cage type coil is relatively small, and thus a big problem with respect to start-up will not occur. For such reasons, depending on the phase of an applied power source, a significant difference occurs in the starting torque that can be generated.